Campaigners had been concerned that the slaughtered animal was a protected blue whale, the largest species on the planet.
Now DNA has shown it to be the offspring of a blue and a fin whale, as the whaling company had claimed.
Researchers say these hybrids are rare and trading their meat is illegal.
Photographic evidence from anti-whaling groups had shown a large animal being butchered in Iceland early in July – based on these images, some experts concluded that it was a juvenile male blue, a species that hasn’t been deliberately killed since 1978.
Now tests carried out at Iceland’s Marine Research Institute have confirmed that it was the offspring of a female blue whale and a male fin whale.
Why does the species matter?
The key reason for interest in the species was to determine whether this killing was legal or not under Icelandic law.
Weighing as much as 200 tonnes and stretching up to 30 metres, blue whales were hunted to the brink by commercial whalers from many countries including the UK from the 1940s to the 1960s when they became a protected stock under the International Whaling Commission.
That means that all countries, including Iceland agreed not to kill the creatures.
It’s different for fin whales. While there is an international moratorium on killing all whales, Iceland doesn’t agree that fin whales are threatened and gives permits for their hunting.
Hybrids between fin and blue whales are a grey area, say specialists.
Are hybrid blue/fin whales common?
Specialists believe that hybrids are not very common in the waters off Iceland.
“Since 1983, they’ve only recorded five of them,” said Astrid Fuchs from the charity, Whale and Dolphin Conservation.
“Four of them have been killed by whalers and one is a very beloved whale watching object and is still alive – they are very rare,” she told BBC News.
What have the whalers said?
The company involved was certain all along that the animal it killed was not a blue whale but a hybrid.
“I am absolutely confident that it’s a hybrid,” Kristján Loftsson, who runs Hvalur hf, told BBC News at the time.
“To mistake a blue whale for a fin whale is impossible, this whale has all the characterisations of a fin whale in the ocean. There are a lot of blue whales off the Iceland coast, when we see the blows and sail to it, and we realise it is a blue and then we leave it and go and look for fin whales.”
What are campaigners saying now?
Those opposed to whaling say that it doesn’t really matter that the animal was a hybrid and not a blue whale. They are calling for an immediate end to hunting the species in the waters off Iceland.
“The killing of a blue/fin whale hybrid demonstrates the difficulty for whalers at sea to identify which species they are pursuing,” said Sigursteinn Masson from the International Fund for Animal Welfare.
“The result is that a rare and protected species has suffered as collateral damage from a cruel, unnecessary and increasingly unpopular hunt.”
What will happen to the whale meat?
Iceland sells almost all of its whale meat to Japan; one of a handful of countries that reject the international consensus to protect whales. Now that this whale has been confirmed as a hybrid it means the meat can’t be legally shipped anywhere.
Under the international regulations that govern animal trading, it is the protected status of the hybrid parents that matter – so as it has blue whale parentage, the Japanese market would be closed to it.
What are the implications for Icelandic whaling?
As this whale has been shown to be a hybrid, it is likely there won’t be major repercussions for the whalers.
Mr Loftsson says he is being targeted by campaigners and there is nothing unusual about the recent killing.
“This is nothing new to us, we have had at least five in previous years with similar characteristics and DNA analysis shows a completely different profile from a fin whale and that has been described as a hybrid of a blue and a fin,” he told BBC News.
Campaigners, though, believe it could still be the beginning of the end.
“We hope it might be the nail in the coffin of Icelandic whaling,” said Astrid Fuchs.
“It confirms what scientists have been saying for years, whaling can’t be regulated – it is always a bit out of control, they are going out there but they don’t know what they are shooting.”